The present invention relates to code division multiple access (xe2x80x9cCDMAxe2x80x9d) communications and navigation systems and more particularly to a hop overlay signal structure generation for a global positioning system.
Due to the general lack of available frequency spectrum, there is a need to provide additional signaling capability by means of sharing frequency bands. For example, the current signal structure for the Global Positioning System (GPS) consists of two downlink L-Band signals, designated L1 and L2. L1 is Quadrature Phase Shift Key (xe2x80x9cQPSKxe2x80x9d) modulated with two separate codes, the C/A code (for general use) and the P(Y) code (for authorized use only).
The P(Y) code is extremely long and requires either a precise timing reference or handoff from the C/A code. Conventional military receivers acquire the C/A code first, and then transition to the more precise P(Y) code for their navigation solution. Currently, L2 only contains the P(Y) code, and is hence for authorized use only. As is known to those possessing ordinary skill in the art though, full navigational accuracy requires dual frequency downlinks in order to remove the effects of the ionospheric delay, for example.
The additional capability that is required for civilian users is the provision of a second and, possibly a third, civilian frequency, such that accurate ionospheric correction is possible for these users as well. Since additional L-Band spectrum is at a premium, L2 has been designated as one of the additional civilian frequencies. Adding a C/A code to L2, making it identical to L1, would provide additional civilian capability. However, the military requires the ability to prevent hostile use of GPS in specific geographic regions and requires the ability to protect friendly use of GPS in highly jammed environments.
This requirement can be met with the inclusion of an additional military signal on L2 and/or on L1. This new signal preferably has substantial anti-jam (AJ) characteristics, such that authorized users can still acquire it when the C/A codes are jammed, while having minimal effect on the existing C/A codes and P(Y) codes.
New signal structures under consideration by the Air Force include an additional wideband, direct sequence, spread spectrum signal to serve as the new military signal. One of the signal structures under consideration includes the xe2x80x9ctricode hexaphasexe2x80x9d arrangement wherein the new military signal is encoded with a Manchester code, such that it has a null at the carrier frequency so as to lessen the interference with the C/A code.
This is a variant of the more general scheme known as Binary Offset Carrier or BOC. BOC signals are generated by multiplying a conventional pseudo random (PN) sequence by a square wave in order to move the spectral peak away from the carrier. This lessens the mutual interference with the C/A code. The resultant waveform has 2 spectral peaks on either side of the carrier, wherein the offsets from the carrier are equal to the square wave frequency and the width of each at the two main lobes is determined by the chipping rate of the PN sequence.
Another scheme, the xe2x80x9cSpilker signalxe2x80x9d, splits the C/A code into two spectral components that are positioned on either side of the carrier, each centered at the P(Y) code nulls.
It should be readily recognized, that although these schemes for sharing spectrum attempt to minimize mutual interference between the various signals by offsetting the spectral peaks of the various signals, the residual interference when three wideband signals share the same band is sufficient to compromise overall system performance.
A new signal format that allows sharing of spectrum with CDMA signals with a minimum amount of mutual interference. A particular application of this concept provides a method for enabling use of a global positioning system including a plurality of satellites each of which transmit at least a first navigational signal on a plurality of channels and a second navigational signal on at least one of said plurality of navigational channels, wherein acquisition of said first navigational signal is typically required for acquisition of said second navigational signal, in an environment sufficiently jammed to inhibit effective acquisition of said first navigational signal, said method including the step of coherently frequency hopping a narrow band signal including sufficient information to enable direct acquisition of the second navigational signal and adapted to minimize interference with the first and second navigational signals over the channel bandwidth.